The present invention relates to microaccelerometers and methods for fabricating same.
Sub-micro-gravity accelerometers are used for measurement of very small vibratory disturbances on platforms installed on earth, space shuttles, and space stations as well as geophysical sensing and earthquake detection. However, the available systems are bulky, complex and expensive, and consume a lot of power. See, for example, Space Acceleration Measurement System (SAMS), http://microgravity.grc.nasa.gov/MSD/MSDhtmlslsamsff.html.
Due to the low-cost and high volume demand, the majority of commercially available microaccelerometers have been developed with low to medium range sensitivities. However, in the past few years, there has been an increasing demand for low-power and small form-factor micro-gravity (micro-g) accelerometers for a number of applications including vibration measurement and earthquake detection. High-performance digital microelectromechanical system (MEMS) accelerometers may also be utilized in ultra-small size for large-volume portable applications such as laptop computers, pocket PCs and cellular phones.
Despite the substantial improvements in micro-fabrication technology, which have enabled commercialization of low to medium sensitivity micromechanical accelerometers, the high precision (<10 μg resolution) accelerometer market has not been dominated by micromachined devices. Moreover, there has been an increasing demand for low-power and small footprint MEMS accelerometers with high sensitivity and stability for many applications such as oil exploration, gravity gradiometry, and earthquake detection. Inexpensive mass-production of these sensitive devices in small size not only can target all these existing applications but also could open new opportunities for applications never been explored with today's available bulky and complex measurement systems.
To achieve the overall device resolution in the sub-μg regime, both mechanical and electronic noises must be extensively suppressed. The dominant source of mechanical noise is the Brownian motion of air molecules hitting the circumferential surfaces of the small micromachined device. Increasing the inertial mass of the sensor is the most effective way of improving the device performance. One implementation of this approach using the full thickness of the silicon wafer combined with high aspect ratio sense gaps has been demonstrated and proved viable in realization of micro-gravity micromechanical accelerometers. Narrow sense gaps in these multiple-mask double-sided processes are defined by a sacrificial oxide layer, which is removed in a wet oxide-etch step referred to as a release step. Considering compliance of the structure required for high intended sensitivity, the sensitivity of the device is limited by the stiction in the wet release step.
The present inventors have previously disclosed 40 μm thick SOI accelerometers with 20 μg/√Hz resolution and sensitivity on the order of 0.2 pF/g. See B. Vakili Amini, S. Pourkamali, and F. Ayazi, “A high resolution, stictionless, CMOS-compatible SOI accelerometer with a low-noise, low-power, 0.25 μm CMOS interface,” MEMS 2004, pp. 272-275. These accelerometers, however, do not have the structure or resolution capability of the present invention.
U.S. Pat. Nos. 6,287,885 and 6,694,814 disclose silicon-on-insulator devices designed as acceleration sensors. However, U.S. Pat. Nos. 6,287,885 and 6,694,814 do not disclose or suggest construction of an accelerometer having added seismic mass or the use of doped polysilicon to reduce capacitive gaps.
It would be desirable to have microaccelerometers that have improved submicron-gravity resolution.